The invention relates to an electronic trip device comprising:
a processing unit connected to current sensors and supplying a tripping order when the current supplied by the sensors exceeds a preset threshold for a preset time, and PA1 at least one setting device, with several positions, connected to the processing unit.
State-of-the-art electronic trip devices comprise current sensors, a processing unit and a trip relay enabling the contacts of a circuit breaker to be opened. The processing unit receives signals supplied by the current sensors and can produce a tripping order to operate the relay. The tripping order is produced when the value of the current signals exceeds preset thresholds for preset times. The thresholds, time delays and choice of type of tripping curves can be fixed, or preferably adjustable by means of setting devices connected to the processing unit.
It is well known to use potentiometers or variable resistances to perform adjustments, notably in analog electronic trip devices, but these components are costly and not very reliable. Digital trip devices preferably comprise selector switches with several positions with encoded single contacts or multiple contacts, like encoding thumb-wheels. The inputs and outputs of the selector switches are controlled by logic circuits, for example by a microprocessor. The circuits determine the settings of the thresholds, time delays and/or choice of type of tripping curves according to the positions of the selector switches.
When adjusting the setting of a selector switch, a user used to progressive adjustments with a potentiometer is liable to position the selector switch between two fixed positions. In the case of a single-contact selector switch, the micro-processor can detect an anomaly and set the adjustments to minimum values or to default values. For example, if two positions are detected, it chooses the lower of the two, and if no position can be detected it sets the adjustments to the minimum.
With a multiple-contact selector switch, such as an encoding thumb-wheel, the closings and openings of the selector switch, contacts follow a known binary encoding method using, for example on 8, 10 or 16 positions. The binary encoding is by increasing value and does not enable the circuits of the processing unit to detect wrong or random values produced by intermediate positisons .